The invention is in the field of composite materials, in particular composite materials which contain eucryptite particles capable of undergoing transformation between the beta (β) and epsilon (ε) phases. The invention provides composite materials which are capable of transformation toughening behavior, methods for making these materials, and methods for transformation toughening of materials.
Crystalline β-eucryptite (LiAlSiO4) is a stuffed derivative of β-quartz (space group P6422 or P6222) where Al3+ ions replace half of the Si4+ ions and charge is compensated by the addition of Li+ ions (Palmer 1994; Buerger 1954; Winkler 1948; Schulze 1972; Tscherry 1972a; Tscherry 1972b; Pillars 1973; Guth 1979). The Li+ ions take different positions within channels of the c-axis (Palmer, 1994), leading to minor structural variations that are very difficult to detect experimentally, but are believed to be important in governing the physical properties. The Li+ ions may be viewed as ‘propping open’ the framework, thereby stabilizing the relatively open β-quartz structure. FIGS. 1a and 1b illustrate two schematics of the structure. In both figures the darker tetrahedra represent SiO4 and the lighter ones represent AlO4. Li cations are represented by the small spheres. In FIG. 1a a view parallel to [110] is shown. (from Sprengard 2000). In FIG. 1b the view is projected along a-axis (from Zhang 2005). Two different Li positions along the channels parallel to the c-axis are indicated, and the unit cell is marked by the solid rectangle. The framework of β-eucryptite may be described as two helical chains of SiO4 and AlO4 tetrahedra that spiral around a 6-fold screw axis (Xu 1999a; Palmer 1994). The SiO4 and AlO4 tetrahedra positions alternate within the (001) layers, and the Li is hence ordered within two distinct channels. Due to the ordering of the cations, a superstructure exists (doubled translational periodicity along the a and c axes) that does not occur in β-quartz. This highly anisotropic framework leads to highly anisotropic physical properties for β-eucryptite. As the stoichiometry changes to Si-rich, the ordering fades away and the superstructure is eventually lost (Xu 1999a). Disorder is also induced at higher temperatures.
Because of its highly anisotropic structure, β-eucryptite is a fascinating material in regards to its physical properties. For example, ordered β-eucryptite exhibits an average coefficient of thermal expansion (CTE) that is negative; basically, the expansion along the a-axis (˜8×10−6/° C.) is more than cancelled by the contraction along the c-axis (˜18×10−6/° C.). As another example, the compressibility of β-eucryptite is highly anisotropic: it is largely incompressible along the c-axis, but is compliant parallel to the (001) plane. Its negative CTE has attracted much attention over the years from a fundamental and engineering point of view. The discovery of thermal contraction in β-eucryptite was made by Hummel in 1951 (Hummel 1951), and the reason for it was subsequently studied by a number of researchers (Hortal 1975; Gillery 1959; Tien 1964), and finally attributed to Si/Al tetrahedral deformation (Palmer 1994). More recently, this unusual behavior has been attributed also to two other processes (all of which are interdependent): 1) Li positional disordering and 2) tetrahedral tilting (Xu 1999a; Phillips 2000; Xu 2001). Because of these unique structural characteristics, some of the fundamental studies on LiAlSiO4 materials have been conducted simply because they provide insight into general crystal-chemical systematics (Palmer 1994; Xu 1999b). It is also noted that β-eucryptite exhibits anisotropic superionic conductivity with the transport of Li+ ions parallel to the c axis channels, hence its interest in lithium-based batteries.
There exist two well-established polymorphs in addition to β-eucryptite: 1) α-eucryptite which exists over a range of temperatures that depend on the exact stoichiometry, and is stable at ambient conditions, but is typically kinetically hindered (Xu 1999; Beal 1994); and 2) γ-eucryptite, a metastable phase relatively recently discovered (Dondu 1986). It has been shown that β-eucryptite is no longer stable at ambient temperatures with less than about 35 mol % substitution of Li++Al3+ for Si4+ (Xu 2000); in that case α-eucryptite forms. The critical temperature of the β-α transformation depends on the relative (Li+Al)/Si concentration.
Recently, a third polymorph, eucryptite, has been discovered (Zhang 2002). In that study, in-situ compression experiments at ambient temperature, revealed that β-eucryptite transforms reversibly to an orthorhombic phase, which the authors termed ε-eucryptite, starting at pressures of 0.83 GPa (Zhang 2002). If heated to temperatures above 600° C., while under pressures above 0.83 GPa, it transforms irreversibly to α-eucryptite (Zhang 2002). More recently, it was observed that at pressures above about 5 GPa it begins to amorphize until about 17 GPa, above which it is completely amorphous (Zhang 2005). The fact that this pressure is relatively low for pressure-induced amorphization in most materials (Sharma 1996) is believed to be due to the relatively open framework structure of β-eucryptite (FIGS. 1a and 1b). Interestingly, release of the pressure before it reaches 17 GPa leads to re-crystallization of β-eucryptite, indicating the presence of a kind of structural memory (Zhang 2005). On the other hand, exposure to pressures above 17 GPa leads to a complete and irreversible amorphization. The theoretical density of β-eucryptite is about 2.34 g/cm3, whereas that for eucryptite is about 7.7% higher at 2.52 g/cm3 (Xu 2005).
FIG. 2 depicts the sequence of these transformations. In FIG. 2, the symbol “a” represents the amorphous phase. It is believed that only one partial pressure-temperature diagram for this system exists to date (Zhang 2005). Two studies documenting pressure-induced transformations of β-eucryptite to ε-eucryptite or the amorphous phase are those by Zhang et al. (2002, 2005). It is noted that pressure induced transformations have been observed in other minerals with the relatively open β-quartz structure, but in most cases, the transformation is amorphization (Richet 1997; Huang 1998; Secco 1999). Other than d-spacings and lattice parameters from x-ray diffraction data (Zhang 2002), nothing is believed to be known about the crystal chemistry of ε-eucryptite, and the only properties believed to be measured are the coefficients of thermal expansion.
Composites combining eucryptite with other materials have been reported, the composites typically have small positive or negative coefficients of thermal expansion. U.S. Pat. No. 6,566,290 to Beall et al. reports a biphasic ceramic having a first phase of beta-eucryptite and a second phase of a higher temperature phase such as lithium aluminate spinel, lithium aluminate, corundum, and combinations thereof. The composites have at most 50% beta eucryptite and an open porosity between 35-65% by volume. Shimada et al. (1996) report composites made from β-eucryptite (5 micron average initial diameter) and yttria-stabilized partially stabilized zirconia (PSZ) (crystallite size 40 or 24 nm). Shimada et al. explored compositions with 35 or greater wt. % beta-eucryptite (because beta eucryptite is less dense than zirconia, the volume percents were greater than 35%). Japanese Publication Nos. 2001-302338 and 2001-302339 report composites of beta eucryptite and silicon carbide and/or silicon nitride. U.S. Pat. No. 5,147,829 to Hench et al. report sol-gel derived SiO2/oxide powder composites, where the oxide powder can consist of beta eucryptite crystals.
U.S. Pat. No. 4,806,704 to Belke, Jr. et al. report aluminum matrix composites with beta-eucryptite as the additive in a volume percent up to 60%. European Patent Publication EP0217176 reports composite materials made of metallic and nonmetallic components. The metallic component can be 20-80% copper and/or molybdenum and the nonmetallic component can be beta eucryptite. Wang et al. (2002) report a composite of β-eucryptite particles (5-10 micron diameter) and aluminum borate whiskers (diameter 0.5-1 micron and length 10-30 micron) in 6061 aluminum alloy. The total volume fraction of reinforcement was 40% and the volume ratio between particles and whiskers was 2:1.